Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual é a soma dos headcounts que têm um valor n/a de $60k a $70k?
 ###Tabelas: CREATE TABLE faculty_and_staff (faculty_and_staff_id AUTO_INCREMENT PRIMARY KEY, Data VARCHAR, Headcount INT, Percent of total VARCHAR, Less than $25,000 VARCHAR, $25,000 to $40,000 VARCHAR, $40,000 to $50,000 VARCHAR, $50,000 to $60,000 VARCHAR, $60,000 to $70,000 VARCHAR, $70,000 to $80,000 VARCHAR, More than $80,000 VARCHAR, Mean VARCHAR) CREATE TABLE jersey_progress_chart (jersey_progress_chart_id AUTO_INCREMENT PRIMARY KEY, Stage (Winner) VARCHAR, General Classification Yellow Jersey VARCHAR, Mountains Classification Green Jersey VARCHAR, Points Classification Navy Blue Jersey VARCHAR, Intermediate Sprints Classification Red Jersey VARCHAR, Team Classification VARCHAR) CREATE TABLE list_of_aftershocks (list_of_aftershocks_id AUTO_INCREMENT PRIMARY KEY, Date (YYYY-MM-DD) VARCHAR, Time ( UTC ) VARCHAR, Latitude VARCHAR, Longitude VARCHAR, Depth VARCHAR, Magnitude VARCHAR) CREATE TABLE itf_circuit_doubles:_9_4–5 (itf_circuit_doubles:_9_4–5_id AUTO_INCREMENT PRIMARY KEY, Outcome VARCHAR, Date VARCHAR, Tournament VARCHAR, Surface VARCHAR, Partner VARCHAR, Opponent in the final VARCHAR, Score in the final VARCHAR) CREATE TABLE facelift (facelift_id AUTO_INCREMENT PRIMARY KEY, Model VARCHAR, Engine VARCHAR, Displacement VARCHAR, Valvetrain VARCHAR, Fuel system VARCHAR, Max. power at rpm VARCHAR, Max. torque at rpm VARCHAR, Years VARCHAR)  
SELECT SUM Headcount FROM faculty_and_staff WHERE $60,000 to $70,000 = n/a